In general, most conventional boilers employ a one-directional-combustion-type heat transfer mechanism. An example of that boiler is, as shown in FIG. 23A, constructed such that a burner 104 is installed at one end of a combustion chamber, from which burner, a combustion gas is blown, and during discharge of the gas from another end of the same combustion chamber, a heat of the gas is transferred to water in the boiler tube by means of a radiation heat transfer and a convection heat transfer, to thereby generate steam. The combustion chamber is divided into a radiation heat transfer section 101 for providing radiation heat to the boiler water, and a convection heat transfer section 102 for providing convection heat to the boiler water. In the radiation heat transfer section 101, although both flame temperature and heat absorption of the boiler are high (see FIG. 23B), the heat from the combustion gas is not completely transferred to the boiler water, which thus requires installing the convection heat transfer section 102 at the downstream site in order to recover the combustion gas heat. In the absence of such convection heat transfer section 102, the temperature of the exhaust gas will increase, giving a bad influence on the environment and also lowering thermal efficiency.
There has been known a boiler of the type which superheats a saturated steam in a superheater section provided therein. For example, as shown in FIG. 24A, in this sort of boiler, a superheater section 103 is defined between a radiation heat transfer section 101 and a convection heat transfer section 102. With this arrangement, a saturated steam generated in both radiant and convection heat transfer sections 101, 102 is superheated in the superheater section 103 so as to obtain a superheated steam. Alternatively, as in FIG. 25A, some of boilers of this sort have the superheater section 103 defined behind the radiant and convection heat transfer sections 101, 102 so that a saturated steam generated therein is superheated at the extreme downstream end of the combustion gas path. In those boilers, a typical way to adjust a temperature required for the superheating is by providing a bypass which does not pass through a heat exchanger in the superheater section 103, thereby controlling the amount of combustion gas flowing through the superheater section 103, or by cooling the superheated steam by way of a heat exchange with the boiler water and the like or by means of a cooling water being sprayed in the superheated steam.
However, the convection heat transfer section 102 is undesirably low in terms of a heat absorption of the boiler, resulting in the need for increase of the heat transfer area of that particular section in an attempt to recover heat sufficiently and raising the problem that the furnace thereof also requires larger dimensions. Further, the conventional furnace employed in the boiler has been found defective in that the heat absorption of the boiler in the radiation heat transfer section 101 is changeable extraordinarily, which does not provide a uniform heating of a boiler water flowing in the water tubes or which may become full of steam. To prevent this problem, it is necessary to insure the circulation of the boiler water, and therefore, another problem is posed: The dispositions of heat transfer tubings are limited, and a pump and power are required for executing a forced circulation of the boiler water that need more initial and running cost.
In this conventional boiler with superheater, furthermore, there has been such problem that its damper or duct is damaged by a high temperature of combustion gas flowing in the bypass, an that, in the case of lowering a temperature by spraying water in the superheated steam, a thermal efficiency will become poor. Still further, to dispose the superheater section 103 behind the convection heat transfer section 102 will also make poor the thermal efficiency, though it may obtain a low-temperature superheated steam. In addition thereto, there will be the problem that if the temperature of superheated steam is lower than a predetermined temperature, a proper improvement will be needed for increasing the heat transfer area of the superheater.